This application is a 371 of PCT/GB98/01696 filed Jun. 16, 1998.
1. Field of the Invention
The invention relates to a method of treating glass suitable for incorporation in a glass production process and in particular to a method involving polishing especially coated glass. The invention also relates to polished coated glass and glazing incorporating such glass, and a float glass production line incorporating a polishing station.
Glass made by the float glass production process is commonly known as float glass. The float process involves forming molten glass into a continuous ribbon by floating the molten glass on molten tin and then conveying it through an annealing lehr at line speeds that may be 1000 m/hour or higher. For some time, coatings have been applied to float glass on-line, that is, during the glass forming, i.e. glass production, process. A coating may be deposited on to the ribbon pyrolytically, whilst it is still hot. This may be done in the molten tin bath, in the annealing lehr or between the two. Chemical vapour deposition is one pyrolytic technique which is known to be used in the on-line coating of glass.
An example of an on-line coated glass is the applicant""s low emissivity (infra-red reflecting), fluorine doped tin oxide coated glass (hereinafter referred to as xe2x80x9cPILKINGTON K GLASSxe2x80x9d which is the applicant""s trade mark for the glass). Low emissivity glass is commonly used in multiple pane insulating glazing units. The low emissivity coating reflects infra-red radiation, which would otherwise escape through the unit, back into the room.
PILKINGTON K GLASS(trademark), because of its well regarded performance capability, is now widely used in multiple glazing. However, as part of a continual process of upgrading, the applicants have been addressing what further improvements may be made to the performance of the glass and specifically its haze and roughness characteristics.
An advantage of PILKINGTON K GLASS(trademark), and other glasses with coatings applied pyrolytically on-line is that they are usually extremely hard. Coatings applied off-line, that is away from the float glass production line, say using separate DC magnetron sputtering apparatus, are commonly soft. Certain types of glazing, for example secondary glazing, have all their surfaces on occasion exposed and accessible. The hardness and hence durability of on-line coatings makes them ideally suited to use in such types of glazing. The less durable soft coatings, on the other hand, are limited to use in glazing applications in which they can be protected from exposure and accessibility and the consequent deterioration they may suffer. Thus, hard coatings tend to offer considerably more versatility than soft coatings in terms of glazing applications, although care is needed when handling, packing or cleaning glass with a hard coating so as not to leave marks upon it; because of their hardness, on-line coatings may abrade metal objects, such as wedding rings, and visible marks may be left upon them. Other instances when hard coatings are preferred to soft coatings are: double glazed roof lights which ideally have an infra red reflecting coating on the exterior-most surface so as to prevent icing and/or condensation formation; single glazed oven doors with an infra red reflecting coating on the inside surface so as to prevent heat escaping and to minimise the door surface temperature; insulating glazing units have been found to operate most effectively with low emissivity coatings on surfaces one and three (conventionally the pane surfaces of insulating glazing units are numbered one to four from the exterior-most surface to the interior-most surface).
Under particular, though not altogether common, lighting conditions, light scattering at the coating surface may cause certain on-line coated glasses to appear very slightly hazy. Slight haziness may be the result of any one or more of the roughness of the coating (which may be dictated by the deposition method, particularly the temperature employed) the coating precursors used and the coating thickness.
It is also known that the surface quality of uncoated glass, typically rolled wired glass, can be improved by grinding and polishing, but any polishing is done off-line with rotating polishing heads whose axes of rotation are perpendicular to the plane of the glass. Such perpendicular polishing arrangements tend to involve a considerable number of large and complex mechanisms which may be expensive, and successfully engineering such mechanisms into a float line would involve surmounting a number of problems. In particular, the formed glass ribbon is transported along a float line on spaced rollers whereas a perpendicular polishing arrangement traditionally requires the glass to be supported on a continuous flat bed. Also, the formed ribbon of float glass has selvedges which a perpendicular polishing arrangement could not easily accommodate.
2. Description of the Related Art
In United Kingdom patent application GB 2 196 349A a method of producing a vehicle window is disclosed which comprises depositing a coating on the surface of a glass sheet or ribbon, with the coating afterwards being polished using a battery of foam pads which apparently reciprocate over the glass surface; it is not clear whether the polishing treatment is carried out on-line or off-line. However, the description does not disclose how a fast polishing process which is capable of effectively polishing a glass ribbon at line speeds of, for example, 1000 m/hour or higher may be carried out, while ensuring that the glass ribbon is polished uniformly across its width, without visually noticeable areas of different polishing quality (although haze in unpolished PILKINGTON K GLASS(trademark) and other hard coats is not usually readily apparent, adjacent areas of PILKINGTON K GLASS(trademark)polished to different levels of roughness are much more apparent to the eye).
The invention provides a method of making coated glass comprising forming a continuous ribbon of glass, depositing a coating on to a surface of the ribbon during the production process and polishing the coated surface of the ribbon, using at least one rotating polishing head.
It has been found that on-line coated glass which is also polished on-line in accordance with the invention has significantly improved haze characteristics and improved resistance to marking. Resistance to marking is of particular benefit in glazing applications where the position of the glass will mean that the coating is on an exposed surface. On-line polishing of coated glass also has a large cost advantage over off-line polishing which would require the additional expense of at least handling and transporting the glass from the float line to a suitable polishing rig. In addition, on-line polishing may reduce the so-called micro-corrugations present in the surface of some of the thinner float glass which is coated for use, generally, in electronics applications. On-line polishing is also useful in the production of thicker-than-normal coatings (in the sense of thicker than would most commonly be used for glazing applications): a thicker-than-normal coating with improved emissivity is initially deposited and then polished so as to reduce haze.
Preferably, the coated surface is polished with one or more rotating polishing heads having an axis of rotation substantially parallel to the plane of the glass. Further preferably, the axis of rotation of the or each head is substantially parallel to the transverse axis of the ribbon. Adjacent heads may rotate either in the same or an opposite direction. The or each head may extend partially across the width of the ribbon, with one or more heads extending across one or both selvedges of the ribbon; the heads may be staggered so that in combination a plurality of part-width heads cover the full width of the ribbon; the heads may reciprocate along the transverse axis of the ribbon and may comprise a plurality of mops, preferably disc-shaped, mounted on a mandrel. The mops may be of cotton or of any other similar or suitable material especially soft materials. Disc-shaped mops are preferably up to 750 mm in diameter. Polishing medium may be applied to the or each head at the interface(s) between coating surface and polishing head, for example, by forming a pool of polishing medium on the coated surface. Alternatively, the mops may be impregnated with a polishing medium. The polishing medium may be alumina- or silica-based (although use of a silica based polishing medium is not preferred for polishing tin oxide coated glass as it is believed to increase the risk of scratching), pumice, tin oxide or any other equivalent or similar medium suitable for the purpose.
The glass may be supported underneath by rollers, which may be driven so as not unduly to stress the ribbon, or by any other support means which is capable of supporting the glass in such a way that the polishing is achieved evenly and without excessively distorting the formed ribbon.
The coating may be any material capable of changing or enhancing the properties of the glass in some way, but the invention is especially useful for polishing coating surfaces comprising metal oxide, preferably semiconducting metal oxide, such as fluorine, antimony or indium doped tin oxide.
While the invention has been developed primarily for the on-line treatment of coated glass, it has been found that the use of one or more polishing heads rotating about an axis parallel to the plane of the glass surface offers important advantages over prior art methods for polishing glass (whether coated or otherwise) off-line using polishing heads rotating about vertical axes. A surprisingly high polishing rate may be achieved, with a low residence time of the glass under the polishing head (enabling, for on-line polishing, the polishing station to be accommodated in a much smaller space on the line than anticipated if using a conventional off-line glass polishing procedure with polishing heads rotating about vertical areas). Moreover, it is easier to achieve uniform polishing across the width of the glass surface to be polished, the equipment is cheaper, and not so critical that the glass be maintained exactly horizontal to achieve uniform polishing.
Thus the invention also provides a method of treating glass comprising polishing the glass with at least one rotating polishing head, wherein the axis of rotation of the polishing head is substantially parallel to the plane of the glass.
Not only is the use of a rotating polishing head with an axis of rotation parallel to the plane of the glass advantageous for off-line polishing, as well as on-line polishing of coated glass, it is also useful for on-line polishing of uncoated glass, for example, to remove attached debris or inclusions (known as xe2x80x9ctop speckxe2x80x9d) from the surface of the glass prior to applying a coating.
Whether polishing a coated or uncoated surface, polishing medium may, as noted above, be applied directly to the polishing head(s), or to the interface(s) between the surface being polished and the polishing head(s).
In the former case, it is usually necessary to apply polishing medium to the head with a high pressure system so that the polishing medium is forced deep into the soft mops that comprise the polishing head to limit the extent to which polishing medium on the surface of the rapidly rotating polishing head is thrown off by centrifugal force during the polishing process. To prevent blocking of the spray apparatus, the polishing medium will usually contain grease or other lubricating agent, which, if it collects on the glass, may then need cleaning off.
In the latter case, polishing medium may be applied at the interface of the surface being polished and the head by a low pressure spray or through a pipe, and in neither method of application will grease normally be required in the polishing medium.
In either case, it is usually preferred for the polishing medium to comprise an abrasive, as discussed above, in suspension in a liquid, normally water. The water is believed to lubricate the polishing, reducing the incidence of scratching and formation of dust.
Preferably, the polishing head is dressed at intervals to ensure uniformity. This is important because uniformity of the polishing head improves uniformity of the polished ribbon. Dressing may comprise rotating the polishing head against a rule, including saw or straight edged blades.
A scratch detection device is, preferably, positioned down-ribbon of the rotating polishing head to reduce the likelihood of large areas of glass being damaged by scratching.